1. Field of the Invention
The present invention relates to a symbol read device for optically reading symbols, such as bar codes and characters, by scanning the symbols with a light beam. More particularly the invention is directed to a symbol read device which is capable of reading symbols, regardless of their orientations, by scanning the symbols in different directions.
2. Description of the Related Art
With the widespread use of computers, there has been increasing demand for various types of automated input/output devices. One such device is an optical character reader (OCR) and another is a bar code reader (BCR). Such devices can read characters and codes recorded on cards, packages, and the like. It is required to accurately read characters and codes on character and code bearing surfaces by a read head relatively distant from the surface being read and not in contact with the character and the code bearing surfaces.
A known symbol read device includes a stand-alone type casing containing a laser beam generator for generating a laser beam, a scanning mirror for scanning the laser beam over a predetermined angular range (the scanning mirror could be a polygon mirror or a galvano mirror), a photodetecting element for receiving a light beam reflected from a symbol surface bearing a symbol thereon, and a signal processor for processing an output signal of the photodetecting element so as to recognize the symbol being read.
A known bar code reader employs a laser beam for scanning a surface bearing the bar code. The laser beam forms a small spot even at a distant position from the surface being read. Thus, it is capable of reading bar codes near to or far from the reader (large read depth). The bar code reader can automatically scan bar codes with the laser beam. An operator places an object bearing a bar code within a read field of a laser emitting window and the reader reads the bar code. When using a bar code reader in a pen-type casing, he places and slides a pen point of the device over the symbol surface bearing the code to be read.
In various applications, it is advantageous to use a so called hand-held type bar code reader. The various elements of such a reader are contained in a pistol-like casing for easy use. It can read the bar code by merely pointing the device at a target. It is handy and portable. Further, it can read from a surface of an object with which it is not even in direct contact. For example, a hand-held reader can be used to read a small bar code printed on a printed circuit board.
In a laser scan type symbol read device, laser light repeatedly scans a surface on which there is a symbol to be read. Laser light, having impinged on the symbol laden surface is reflected. The reflected light carries information as to the symbol on the surface. This light is converted into an electrical signal by some sort of photodetector. The output of the photodetector is amplified by an amplifier and signal processed by some kind of recognizing circuit. The recognizing circuit may, for example, convert the amplified signal into a two-value signal and recognize based on some threshold level.
The symbol surfaces may be located at various distances from the reading device, such as, for example, several cm to 1 m. Thus, the intensities of the reflected light received at the photodetector may vary significantly. If the gain of the amplifier is set to a level required for a close symbol surface, the device cannot read a symbol on a surface that is a great distance from the device. If the amplifier gain is set so as to read distant symbol surfaces, the device will not be able to read nearby symbols because the amplifier will be saturated.
For this reason, a symbol read device designed to have a wide read range employs an automatic gain control technique which automatically accommodates the magnitude output of the photodetecting element on the basis of the amount of the reflected light from the previous scan. Levels of signals input to the recognizing circuit are limited to be within a preset range by selecting a small gain for a large quantity of reflected light from a close symbol surface, and a large gain for a small quantity of reflected light from a distant symbol surface. Thus, such a symbol read device can provide symbol recognition regardless of read distance.
Recently, symbol read devices have been developed that employ an omni scan process or a raster scan process to make it easier for an operator to scan symbols. In the omni scan process, as shown in FIG. 13, a symbol is scanned by a plurality of scan lines at different angles. In the raster scan process, as shown in FIG. 14, symbols are scanned by a plurality of parallel scan lines s1 to s5.
The bar code read device of the omni scan type can read the symbol if any one of the scan lines scans across the entire bar code. Accordingly, there is no need to place the scanning device in a particular positional relationship with the bar code being read. The handling of the bar code read device when it reads symbols is remarkably improved.
A raster scan bar code device can read symbols if the scan lines are slightly oblique with respect to the bar code, because any of the scan lines will move across the entire bar code. In this respect, the device is easy to handle for reading a bar code. The device can even read a bar code with a cut-out if any of the scan lines traverses a perfect bar code portion of a defective bar code.
Bar code read devices including a laser beam are generally arranged so as to constantly emit a laser beam. The emitted laser beam may directly hit the operator's eyes. Such devices are usually designed to read a relatively distant bar code. However, when a bar code, which is not the one intended to be read, unexpectedly comes in a read range of the device, it will mistakenly be read. In this respect, it is desirable that the device emits the laser beam only when reading bar codes.
To cope with the problem, a conventional bar code read device includes a read start switch. The laser beam is emitted only when the switch is operated. Of course, the switch must be operated every time the bar code is read. This is very inconvenient particularly in such a situation as to read a number of bar codes, because the switch must be operated many times. Accordingly, repetitive operations impede efficiently entering data.
To solve the problem, there is a technique disclosed in Japanese Patent Application Unexamined Publication No. 64-25287. The publication discloses a laser oscillator for generating the laser beam that is intermittently operated even when a bar code is not being read. The oscillator is continuously operated when an output signal of a photodetecting element is within a preset range of its value during the oscillation of the laser oscillator. When a quantity of the light reflected from the symbol surface is within a predetermined range of its values, the device judges that the symbol surface is present at a bar code read possible position, and automatically starts the operation of reading the bar codes. This technique is efficient.
However, there are problems associated with this technique. First, there is a danger that the laser beam may hit the eyes of the operator or persons present near the bar code read device, because the laser beam is intermittently emitted even when a bar code is not being read. Secondly, an additional controller is required. That is, to detect the symbol surface, the laser beam is directed toward the center of the scanning angular range and fixed thereat. The controller is additionally required to effect the control of the laser beam, which makes the construction of the device somewhat complicated.
The bar code reader of the multi-directional scan type is advantageous in that the positional relationship between the bar code and the bar code reader is not always accurately set. This can effect efficient reading of bar codes. Also, when this type of bar code reader reads a plurality of bar codes closely arranged side by side, it may mistakenly read a bar code that is not intended to be read. A read error occurs when the laser beam obliquely scans a bar code, but fails to entirely scan the bar code. In the multi-directional scan type bar code reader, a read error will more frequently occur than in the unidirectional scan type bar code reader, because in the former, the positional relationship between the bar code and the bar code reader is roughly set. The problem is serious particularly when the data after read is not checked, and use of the unidirectional scan for reading bar codes sometimes ensures a reliable data read. Here, the check is a process of judging as to whether the bar code read is correct or not by detecting the first and last characters of a bar code when these characters are preset or the number of digits of a bar code when it consists of a predetermined number of digits.
As described above, multi-directional scan bar code readers are advantageous in some respects and are disadvantageous in others.
In omni scan and raster scan type bar code readers, the symbol surface is scanned in successive order, so that the scanned position on the surface is different every scan. The quantity of the light reflected from the symbol surface may be different from scan to scan. The change in reflected light may be remarkably large particularly when the photodetecting element receives the light from the symbol surface positioned directly opposite to the photodetecting element or when an object of high reflectivity, for example, metal is present on the scan path.
The automatic gain control technique which determines the gain of a variable gain amplifier on the basis of a previous scan, fails to deal with such a situation. When the amount of received light is extremely increased, the gain for the output of the photodetecting element in the present scan is controlled to be extremely small because of the extremely high quantity of the reflected light in the previous scan. Under this condition, if the symbol is correctly scanned, the read device cannot recognize the symbol since the gain for the output of the photodetecting element is considerably small. Thus, when using the bar code read devices of the omni scan type and the raster scan type, there is a case where use of the automatic gain control technique will degrade read performance. One way to avoid this is to narrow a range of read distance (read range), and to not use the automatic gain control technique. However, this makes it difficult to handle and operate the bar code read device.